Sites that have the HAL module enabled and are upgrading from 8.1.x have encountered #2867444: Configuration schema errors whilst updating to 8.3.0. Possible workarounds include updating to 8.2.7 first and then to 8.3.0, or (for sites using Drush) updating to Drush 8.1.10. (Note that 8.1.x has been end-of-life since September 2016, so we generally do not provide full support for updates that skip minor releases.)

File usage tracking causes unexpected file deletion

Drupal 8 currently has a number of known issues related to incorrect tracking of file usage, which can result in files being unexpectedly deleted from a site. Site owners experiencing unexplained file loss should consider setting "Delete orphaned files" to "Never" on /admin/config/media/file-system until these issues are resolved, and note the following issues: